This invention relates to spring brake actuators as used by large vehicles having pneumatic braking systems which include a power spring and serve as an emergency brake and a parking brake.
Spring brake actuators having a power spring are mandated to actuate the pneumatic braking system of vehicles having large inertial mass, including trucks, buses and tractor-trailers, to brake the vehicle when the pneumatic braking system fails preventing a runaway vehicle. The spring brake actuator also serves as a parking brake when the vehicle is turned off. Such spring brake actuators typically include a cup-shaped housing and a cup-shaped cover or head having a rim portion which is secured to a rim portion of the housing forming a spring or emergency chamber, a reciprocal piston and a power spring located between the cover and the piston which actuates the braking system of the vehicle when the pneumatic pressure in the spring chamber falls below a predetermined pressure. The spring brake actuator is typically combined with a service chamber which actuates the vehicle braking system during normal braking operation, wherein the housing of the emergency chamber is an H-shaped flange case and the service chamber is enclosed by a cup-shaped service chamber housing. Because of the danger of inadvertent opening of the emergency chamber having the power spring, the cover or head is secured to the housing or flange case by permanent defamation as disclosed in U.S. Pat. No. 4,960,036 assigned to the assignee of the present application.
At present, there are two general types of spring brake actuators, including piston-type spring brake actuators and diaphragm-type spring brake actuators. In piston-type spring brake actuators, the piston includes a peripheral seat which seals against the internal surface of the emergency chamber as disclosed, for example, in U.S. Pat. Nos. 5,636,562 and 5,623,863, assigned to the assignee of the present application. Diaphragm-type spring brake actuators include a flexible cup-shaped diaphragm having a peripheral rim portion secured between the opposed rim portions of the cover and the housing which is biased against the power spring piston by a separate spring piston, as disclosed in the above-referenced U.S. Pat. No. 4,960,036. Each type of spring brake actuator has important advantages. Piston-type spring brake actuators have superior performance because of the increased available space for the power spring, permitting the use of a larger power spring. The diaphragm-type spring brake actuator, as disclosed in the above-referenced U.S. Pat. No. 4,960,036, is preferred in many applications because of its reduced cost and size. Both types of spring brake actuators are generally combined with a service chamber having a flexible cup-shaped diaphragm as disclosed, for example, in the above-referenced patents.
The improved spring brake actuator of this invention combines the advantages of diaphragm and piston-type spring brake actuators, including improved performance, reduced cost and reduced size.
The spring brake actuator of this invention includes a generally cup-shaped housing, which may be the upper portion of a flange case, a cover portion having a rim portion secured to a rim portion of the housing, forming an enclosed spring or emergency chamber, a reciprocal piston located within the spring chamber having a peripheral rim portion spaced from an internal surface of the spring chamber, a power spring located within the spring chamber, between the cover and the piston, and a tubular flexible diaphragm having one end portion secured between the rim portions of the housing and the cover and a second end portion received around and secured in sealed relation to the peripheral rim portion of the piston to reciprocate with the piston when the power spring expands to brake the vehicle. In the preferred embodiment of the spring brake actuator of this invention, the second end portion of the tubular flexible diaphragm is inverted, such that the exterior surface of the tubular diaphragm is received around the peripheral rim portion of the piston and the second end portion of the diaphragm is secured to the peripheral rim portion of the piston by a flexible band, such as a continuous wire or nylon cord. In the most preferred embodiment, the interior surface of the second inverted end portion of the diaphragm includes a groove which receives the flexible retaining band and the exterior surface of the piston includes a groove opposite the flexible band which provides further securement. The exterior surface of the inverted tubular flexible diaphragm includes one or a plurality of integral sealing rings or lips which improve sealing of the diaphragm to the piston. Further, in the most preferred embodiment, the exterior surface of the piston which receives the diaphragm is generally frustoconical, further improving the sealing of the diaphragm against the piston.
In the most preferred embodiment of the spring brake actuator of this invention, the piston includes an annular cup-shaped portion which opens toward the cover and receives one end of the power spring and the cover includes a generally radially extending portion which receives the rim of the cup-shaped portion of the piston when the piston is biased against the cover by pneumatic pressure in the emergency chamber. This configuration reduces the stress on the cover and increases the space available for the power spring, permitting the use of a more powerful power spring, thereby improving performance. Further, the cup-shaped annular portion of the piston includes a generally flat bottom wall which nests against the bottom wall of the housing or radial web of a flange case, permitting a longer stroke for the piston. In the most preferred embodiment, the cover includes a cup-shaped portion adjacent the generally radially extending portion, further increasing the volume available for the power spring. The rim portion of the cover is most preferably secured to the rim portion of the housing by an integral skirt portion having a free end which is permanently deformed around a rim portion of the housing and the first end of the tubular flexible diaphragm extends longitudinally and preferably includes an integral bead spaced from the free end which is received within a groove in the housing, providing a secured sealed connection between the housing and the cover.
The improved spring brake actuator of this invention thus combines the advantages of piston and diaphragm-type spring brake actuators, providing improved performance, as compared to conventional diaphragm-type spring brake actuators, and reduced cost and size, as compared to piston-type spring brake actuators. Other advantages and meritorious features of the spring brake actuator of this invention will be more fully understood from the following description of the preferred embodiments, the claims and the appended drawings, a brief description of which follows.